The present invention relates to a satellite radio wave receiving apparatus, a radio controlled watch, a code signal acquiring method, and a recording medium.
Conventionally, there has been used an electronic watch (a radio controlled watch) having a function to acquire date and time information by receiving radio waves including the date and time information, and correct a date and time counted by the watch. The radio controlled watch can maintain substantially exactly counting and displaying a date and time without a user's correction operation by periodically and automatically performing such date and time correction.
The radio wave to be received by such a radio controlled watch includes radio waves transmitted from various positioning systems relating to a global navigation satellite system (GNSS), such as a positioning satellite in a global positioning system (GPS) of the United States. The radio wave from the positioning satellite is receivable in a wide area on the earth where the skies are visibly recognized, and is preferably used to adjust a date and time all over the world. In the radio wave transmitted from a positioning satellite, a signal coded in a predetermined format determined according to a positioning satellite (a code signal; a navigation message) is transmitted, and date and time information is acquired by decoding the code signal in accordance with the format.
By modulating a phase using an individual pseudo-random code for each positioning satellite, spread spectrum is performed to the code signal transmitted from the positioning satellite. In a positioning satellite in a GPS (referred to as a GPS satellite), by using a pseudo-random code sequence (a C/A code) having 1 msec period, phase modulation (spread spectrum) is performed to a code to be transmitted every 20 msecs (50 Hz) with 20 periods of C/A codes. After the C/A code and a phase of the C/A code in the received radio wave are identified, to decode the navigation message, it is required to identify a head position of each code (a code synchronization point), that is, to identify that the code is changed in synchronization with the C/A code in which period among the 20 periods.
In the radio controlled watch, the load of the processing to receive the radio wave is greatly larger than the processing to count and display the date and time. Especially, a portable electronic watch, such as a wristwatch which needs downsizing and lightning, has a problem that the weight limit of the mounted battery makes receiving processing for a long time and repetition of receiving processing in a short time difficult. On the other hand, the identification of the synchronization point sometimes fails when the receiving intensity is low, and has a problem that the processing cannot be terminated in a short time.
In contrast, for example, JP 2007-187462 A discloses a technique to complete identification of a synchronization point in a short time by performing in parallel the identification of the synchronization point of a C/A code used for performing phase modulation (spread spectrum) in a transmission radio wave in a L1 band of a GPS satellite and a P code used for performing spread spectrum in a transmission radio wave in a L2 band, and improving the synchronization accuracy.
However, with a conventional technique, performing multiple identifying processing in parallel increases the processing load. In other words, when a satellite radio wave is received, there is a problem that a code signal cannot quickly certainly be acquired from the received satellite radio wave while suppressing a load caused by identifying a synchronization point.